With the high integration of semiconductor memory elements, the miniaturization of memory cells is proceeding. Use of noble metals such as ruthenium, iridium, and platinum as electrode materials for such memory elements is being investigated. For use in DRAM elements among the memory elements, ruthenium is considered to be a promising electrode material because oxides thereof also have electrical conductivity and ruthenium has excellent suitability for microfabrication. An optimal process for producing a ruthenium-containing thin film in a highly integrated memory element is chemical vapor deposition (hereinafter referred to as CVD) because this method attains excellent step coverage. Organometallic compounds, which are easy to handle among metal compounds, are thought to be suitable for use as raw-material substances for forming thin films using the CVD or atomic layer deposition (hereinafter referred to as ALD). At present, an excellent organometallic compound for forming a thin ruthenium film or thin ruthenium oxide film is (2,4-dimethylpentadienyl)(ethylcyclopentadienyl)ruthenium (hereinafter referred to as DER) because this compound is liquid at ordinary temperature and has excellent handleability and renders stable feeding possible from the standpoints of stability and vapor pressure (patent document 1).
It is generally known that thin-film formation at a low temperature is advantageous to the formation of a thin ruthenium film of good quality by CVD, and there is a desire for a ruthenium compound that has high reactivity at lower temperatures than DER, which has been used hitherto. Non-patent document 1 has reported that bis(2,4-dimethylpentadienyl)ruthenium has high reactivity at lower temperatures than DER. However, since this complex has a melting point as high as 89° C., it is necessary that the complex should be fed after gasified through sublimation. The gasification through sublimation has a problem that not only the rate of carrier gas saturation is low, but also the raw-material gas concentration changes due to a change in the surface area of the solid, making it impossible to conduct stable feeding. A technique in which the complex is used as a solution in an organic solvent has been proposed as a measure for overcoming the problem (patent document 2). However, raw-material feeding by means of bubbling based on the technique has a problem that the solvent only volatilizes because of a difference in volatility between the solvent and the complex, resulting in precipitation of a solid. Consequently, this method is not considered to be an entirely stable method for raw-material feeding. Furthermore, since the dilution with a solvent results in a decrease in the concentration of the ruthenium complex as a raw material, the method brings about a decrease in the rate of film formation. The method hence is undesirable.
Meanwhile, it is known that the formation of a thin ruthenium film by CVD has another problem. That is, there is a delay time (incubation time) between initiation of the feeding of a raw material and initiation of thin-film formation on the substrate. A prolonged incubation time poses problems such as a decrease in the efficiency of thin-film production and difficulties in controlling film thickness. Although it has been reported that DER shows a shorter incubation time than raw materials which were investigated earlier (non-patent document 2), there still is an incubation time. There is a desire for a ruthenium compound which shows a shorter incubation time than DER.